1. Technical Field of the Invention
The present invention relates to a cooled moving blade for a gas turbine, and more particularly to a cooled moving blade formed in such a geometrical configuration that thermal stress induced between a base portion of the blade and a platform can be reduced.
2. Description of the Related Art
FIG. 5 is a perspective view showing a conventional cooled moving blade for a gas turbine. Referring to the figure, a moving blade 1 is mounted on a platform 2 disposed around a rotor (not shown), wherein a cooling air passage 3 is formed inside of the moving blade 1 between a leading edge thereof and a trailing edge in a serpentine pattern that sequentially extends upward and downward in a repetitious and continuous manner. The cooling air is introduced into the cooling air passage 3 from a port located on the inner side of the leading edge of the moving blade 1 by way of a blade root (not shown) portion and is discharged from holes formed in the trailing edge portion of the blade after having blown through the cooling air passage 3. In the figure, reference numeral 4 denotes a curved surface forming a blade surface of the moving blade 1 and numeral 5 designates a fillet ellipse portion R formed in the blade base portion, which will be described below.
FIG. 6 is a schematic diagram showing the portion B shown in FIG. 5 in detail, and more specifically it shows a blade profile of the base portion of the moving blade 1. The base portion of the moving blade 1 is shaped in a curved surface conforming to an ellipse 6, wherein the fillet ellipse portion R 5 is formed so as to extend continuously with a curved surface of the top portion of the moving blade. The elliptical portion mentioned above is formed over the entire circumference of the base portion of the moving blade 1, and the base portion thus has a form that is capable of reducing thermal stress which is caused by high-temperature combustion gas.
Here, it should be mentioned that thermal stress of an especially large magnitude occurs between the base portion and the platform 2. The reason for this can be explained by the fact that since the moving blade 1 has a smaller heat capacity than the platform 2, the temperature of the moving blade 1 increases at a higher rate and within a shorter time period than that of the platform 2 upon start of the gas turbine. On the other hand, the temperature of the moving blade 1 falls at a higher rate and within a shorter time than that of the platform 2, whereby a large temperature difference occurs between the moving blade 1 and the platform 2. This in turn generates thermal stress. Consequently, the base portion is shaped in the form of a curved surface conforming to the fillet ellipse R to thereby reduce the thermal stress.
Recently, however, there is an increasing tendency to use a high temperature combustion gas to enhance the operating efficiency of the gas turbine. As a result, it becomes impossible to sufficiently suppress the thermal stress with only the base portion structure shaped in the form of the above mentioned fillet ellipse portion R, and cracks develop more frequently in the base portion where large thermal stress is induced. Under these circumstances, there is a demand for a structure of the blade base portion that is capable of reducing the thermal stress more effectively.